Don't put "extra-libraries: gmp" in the cabal file; it comes from the buildinfo file

integer.cabal

@@ -28,6 +28,5 @@ Library {
     -- We need to set the package name to integer (without a version number)
     -- as it's magic.
     ghc-options: -package-name integer
-    extra-libraries: gmp
     c-sources: cbits/cbits.c
 }